This invention relates to a method of improving the adhesion of metal applied on the surface of a polyimide, and to articles formed by such a method.
Printed circuit boards typically consist of a sheet of a dielectric, i.e., a nonconductive substrate formed from various filled or unfilled synthetic materials such a phenolics, glass-impregnated epoxies, and the like. The substrate is provided with a pattern of thin metal foil which function as a conductive path on one or both sides. The conductive paths or "traces" are usually formed of a conductive material such as copper, palladium, nickel, gold, and the like. These traces collectively define all of the electrical connections between components on the board, and are routed between appropriate locations on the board.
Plastics such as polyimides are particularly suitable as materials for printed circuit board substrates because of their strength, heat resistance, dimensional stability, and easy moldability. However, metallization does not adhere well to a polyimide surface. The printed circuit, i.e., the plated metal conductive path, can be damaged or separated from the substrate during subsequent manufacturing steps or during use of the circuit board.
Several attempts have been made to increase the adhesion of the conductive metal traces to the polyimide substrates. Adhesion is generally measured as "peel strength", i.e., the force, under controlled conditions, required to peel the metal from the substrate surface. Various surface treatment techniques are often used to physically modify the as-molded polyimide surface and thereby improve the adhesion of metal subsequently applied thereto. For example, polyimide surfaces have been gritblasted to provide a roughened surface which anchors the subsequently-applied metals. Chemical swelling agents or penetrants have been used to swell the surface, and chemical etching agents have been used to remove portions of the surface.
While such methods do increase the adhesion, they are often not entirely satisfactory for several reasons. Such techniques result in degradation of the molecules forming the polyimide surface, and may decrease both the tensile and impact sterngth of the substrate due to swelling and cracking of the entire substrate material, especially in those instances in which the polyimide contains fillers. Grit blasting requires cumbersome equipment and impedes the processing of printed circuit boards.
Another method for increasing adhesion is described in the application of Dumas et al., Serial No. 947,116, filed Dec. 29, 1986, now U.S. Pat. No. 4,775,449, and assigned to the assignee of the present invention. That application teaches the treatment of polyimides with compounds containing the ##STR1## group in order to improve the adhesion of metals subsequently applied thereto.
The primary object of the present invention is to provide a process for improving the adhesion of metal to a polyimide surface without substantial physical modification or degradation of the surface structure.
An additional object of this invention is to provide a photopatternable polyimide surface to which a highly adherent metal layer may be electrolessly applied.